1. Field of the Invention
The present invention relates to a source current measurement apparatus is for applying a source voltage to an electronic device and measuring the source current supplied to the electronic device and a test apparatus using the source current measurement apparatus.
2. Description of the Related Art
In the conventional test of an electronic device such as a semiconductor circuit, a source current supplied to the electronic device used to be measured. For example, a predetermined source voltage is applied to the electronic device, and the source current is measured at that time, so that the quality of the electronic device is judged on the basis of the source current measured.
FIG. 1 shows the configuration of a conventional measurement apparatus 200. The measurement apparatus 200, which supplies a source voltage to an electronic device 300 and measures the source current supplied to the electronic device 300 at that time, includes a power source 202, a resistor 204, an amplifier 206, a resistor 208, a buffer 210, a current amplifier 212, an AD converter 214, an amplifier 216, a resistor 218, and a capacitor 220.
The voltage generated by the power source 202 is supplied to the non-inverting input of the amplifier 206 via the resistor 204. And the current amplifier 212 generates a source voltage corresponding to the voltage outputted by the amplifier 206 and supplies it to the electronic device 300 via the resistor 218. Here, by supplying the AD converter 214 with the voltage difference between both ends of the resistor 218 via the amplifier 216, the source current supplied to the electronic device 300 is measured. And the buffer 210 and the resistor 208 feed back the source voltage supplied to the electronic device 300 to the non-inverting input of the amplifier 206 so as to control the source voltage to be a predetermined value, and the capacitor 220 restricts the change of the source voltage.
In the conventional measurement apparatus 200, as an example of the current amplifier 212, a bipolar transistor is used, where a predetermined direct voltage is applied between the emitter and the collector of the bipolar transistor, the output voltage of the amplifier 206 is applied to the base, and the emitter terminal is coupled to the resistor 218.
FIG. 2 shows the changes of the source voltage VO and the source current IL. As shown in FIG. 2, when the source current IL changes, the source voltage VO changes in response to the change of the current. At this time, it is preferable that the recovery time Δt of the change of the source voltage VO should be small. In the conventional measurement apparatus 200 as described in connection with FIG. 1, however, the source voltage is generated using the amplifier 212, so the recovery time Δt becomes large.
In order to solve the above problem, a voltage controlled amplifier using a capacitor of large capacitance as the capacitor 220 is used in place of the current amplifier 212. Since the capacitor 220 of large capacitance is controlled by the voltage controlled amplifier at a high speed, the recovery time Δt can be reduced.
FIG. 3 shows the configuration of the voltage controlled amplifier. The voltage controlled amplifier includes a pnp transistor 56 and an npn transistor 58 provided in series with resistors 54 and 60 between two different direct voltages (V1 and V2), and voltages corresponding to a reference voltage are applied to the base terminals of the pnp transistor 56 and the npn transistor 58 via a resistor 47, diodes 48 and 50, and a resistor 52 provided in series between the two different direct voltages (V1 and V2). By the above configuration, the source voltage is outputted from the connection point between the pnp transistor 56 and the npn transistor 58.
In the above voltage controlled amplifier, a plurality of diodes or transistors are used, so the deviation in the characteristics of the elements included in the voltage controlled amplifier is liable to occur, and the source voltage cannot be generated with high precision.
And in case that a large source current is supplied to the electronic device 300, the voltage controlled amplifiers may be provided in parallel. In the test of the source current in that case, it is necessary to detect the output current outputted by each of the voltage controlled amplifiers and calculate the sum of the output currents. However, it is undesirable that the circuit size increases due to the operational amplifier circuits. And although it is possible to reduce the increase of the circuit size by detecting the output current of any of the voltage controlled amplifiers and the source current by multiplying the number of the voltage controlled amplifiers provided in parallel, the deviation occurs in the output current of each of the voltage controlled amplifiers due to the element variation or temperature variation described above, so the source current cannot be detected with high precision.